1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concerned with an apparatus and method employed in a nuclear fuel assembly for implementing a spectral shift therein which increases reactivity in the fuel assembly.
2. Description of the Prior Art
In the conventional designs of pressurized water reactors (PWR), an excessive amount of reactivity is designed into the reactor core at start-up so that as the reactivity is depleted over the life of the core there will still be sufficient reactivity to sustain core operation over a long period of time. However, since an excessive amount of reactivity is designed into the reactor core at the beginning of the core life, steps must be taken at that time to properly control it.
One technique to control reactivity is to produce an initial spectral shift which has the effect of increasing the epithermal (low reactivity) part of the neutron spectrum at the expense of the thermal (high reactivity) part. This results in production of fewer thermal neutrons and decreased fission. Then, as fission decreases during extended reactor operation, a reverse shift back to the thermal part of the neutron spectrum at the expense of the epithermal part is undertaken. Such control technique is primarily accomplished through the use of displacer rods. As the name implies, these rods are placed in the core to initially displace some of the moderator water therein said decrease the reactivity. Then, at some point during the core cycle as reactivity is consumed, the displacement associated with these rods is removed from the core so that the amount of moderation and therewith level of reactivity in the core are increased.
One approach considered for removing this displacement is through the use of movable mechanisms, similar to those associated with control rods. Such an approach is described in a U.S. patent application entitled "An Improved Water Displacer Rod Spider Assembly For A Nuclear Reactor Fuel Assembly" by Trevor A. Francis, filed Mar. 20, 1984 and assigned U.S. Ser. No. 595,154 (W.E. 51,560).
Another approach contemplated for removing the displacement is to have membranes provided on the ends of the displacer rods which are penetrated at some point in time to allow the rods to be filled with water. A small heating element surrounding a specially indented end cap on the hollow displacer rod is turned on at an appropriate time. The heat weakens the indented part of the end cap to the point where the external water pressure opens the end cap and fills the rod with water.
A further approach used to remove the displacement is the provision of at least one rod in the fuel assembly filled initially with helium or other suitable gas. Then, as reactor operation proceeds, the gas-filled rod expands and increases in length until it engages a spike mounted on the adjacent portion of the top nozzle. The spike pierces the upper end plug of the rod and permits the rod to fill with water. Such approach is described in U.S. Pat. No. 4,371,495 to Marlatt.
Still another approach to displacement removal is to withdraw water displacer rods at the desired time by using a drive mechanism. This approach is described in U.S. Pat. No. 4,432,934 to Gjertsen et al.
Yet another approach utilizes a rotatable valve disposed in the central opening of a manifold in flow communication with a plurality of water displacer rods. Burnable poison gas initially fills the rods and is slowly released therefrom by operating the valve to progressively open inlet ports to the rods in a sequential pattern. At the same time the poison gas is slowly released from the rods, the displacement is removed by filling the rods with coolant. This approach is described in the first patent application cross-referenced above.
While all of the above-cited prior approaches operate reasonably well and generally achieve their objectives under the range of operating conditions for which they were designed, a need still exists for an alternate approach to the implementation of the spectral shift concept in currently operating nuclear power plants in a cost-effective manner for achieving greater utilization of nuclear fuel being placed in such plants.